1. Field of the Invention
The present invention relates generally to earth-boring bits of the rolling cutter variety. More specifically, the present invention relates to the cutting structure of earth-boring bits of the rolling cutter for mining applications, principally drilling blast holes for recovery of minerals.
2. Background Information
Prior to the advent of the rolling-cone earth-boring bit, bores and boreholes in earthen formations typically were formed with a cable tool or a fixed-cutter drag bit. These early bits were little more than chisels forced into the earth by various means. By comparison the, original rolling-cone rock bit invented by Howard R. Hughes, U.S. Pat. No. 939,759, drilled the hard caprock at the Spindletop field near Beaumont, Tex. with relative ease. This rolling-cone bit was a near-essential part of the drilling and production of oil and gas wells that has propelled the energy industry.
Earth-boring or rock bits of the rolling-cutter variety are also useful in penetrating earthen formations for purposes other than the production of petroleum. These applications generally are referred to as "mining" applications. A principal mining application for mining bits is the drilling of blast holes. Blast holes are relatively shallow (compared to those drilled for production of petroleum) holes in the earth that are used for a variety of purposes, but primarily to insert explosives into the earth for opening subterranean mine cavities.
Rock bits employed in drilling for petroleum generally are run until they are effectively destroyed. Rock bits employed in mining applications generally are not, and may be used to bore or drill more than one hole. Generally, the equipment employed in oil and gas well drilling is more expensive and more expensive to operate than that used in mining operations. Thus, there is a perception that the operational life of a rolling-cone rock bit employed in petroleum drilling applications is a more critical factor than the life of a rock bit employed in mining applications.
Nevertheless, the basic measure of the performance of a rolling-cone rock bit, whether for mining or petroleum drilling applications, is its rate of penetration of earthen formations. A bit that has a long operational life, but drills slowly, has a poor penetration rate, as does a bit that drills quickly, but has a short operational life. Thus, penetration rate measures both the durability and the drilling efficiency of a rock bit.
Many factors affect the penetration rate of a rock bit. One of these factors is the bit's cutting structure, which includes the configuration and arrangement of cutting elements or teeth on the cutters of the bit. For example, bits having steel teeth, milled or formed from the material of the cutter, have generally good toughness and sharpness and are thus useful in drilling relatively soft formations such as clays, shales, soft sandstones and limestones.
For increased durability, the cutting elements are also formed of hard metal inserts or compacts, usually sintered tungsten carbide, which are interference fit or otherwise secured into apertures in the cutter. These carbide cutting elements are more blunt and have lower projections than steel teeth, and thus are adapted for drilling harder formations such as metal ores and igneous rocks.
Even when soft formations are expected, the bit will almost surely encounter hard formation streaks. The cutting structure of a relatively long and sharp-toothed bit can be seriously damaged or destroyed when hard streaks are encountered for any significant period of time. Such damage can adversely affect the penetration rate of the bit even when it encounters soft formations again. Bits having blunter, more durable cutting structures can be used where hard streaks are expected, but the loss of penetration rate through soft material can be significant and adverse.
In addition to the composition of the cutting elements, their individual configurations and arrangement on each cutter can influence the penetration rate of a bit in different types of formation and combination of formations. There is a near-constant need for improvements to the cutting structure of earth-boring bits, for both mining and petroleum drilling applications.